The present invention refers to an optical cable formed of at least one optical fiber inside a means of reinforcement in the form of a metal tube.
In general, optical cables comprise an optical core, in which one or more optical fibers, separate or grouped, for example, into ribbons or similar, are arranged in a housing for said optical fibers, and a covering that provides protection to the optical core, suited to the specific intended application.
Versions are known in which the optical core includes a metal sheath with the optical fibers inside it and the strength members external to it.
British patent 2 176 905 describes, among other things, a cable comprising a plurality of optical fibers loosely enclosed within a metal tube with a C section. The tube, in turn, is surrounded by steel members with high tensile strength.
U.S. Pat. No. 4,371,234 describes a cable comprising a central strength member, formed of nylon-covered steel strands or strands of a fibrous nature, around which a number of polypropylene-coated optical fibers are arranged, in turn surrounded by a polyester wrap and a sheath of other polyester. An aluminum tube, which is a C-section, is fitted over the sheath and then closed, possibly by welding.
U.S. Pat. No. 4,239,336 refers to cables comprising optical fibers enclosed within a tubular electrical conductor, formed in particular of a tube of aluminum with edges butt-welded or of two half-tubes joined together.
British patent 2253717 cites British patent application 2029047, stating that it recognizes the practical difficulty of forming a continuous metal tube without causing damage to the underlying optical fibers and proposes forming the tube from a flat strip, folded around the fibers with edges spaced apart and closing the resulting gap by an overlying pressure resisting layer, thus avoiding the need to weld the edges and the likelihood of damage to the fibers due to overheating. However, the presence of a gap can in some cases result in a region of weakness.
British patent 2253717 describes a manufacturing process for a fiber optic cable in which the optical fibers are enclosed, when they are fed in a longitudinal direction, by a metal strip folded around the fibers and the edges welded together to form a tubular moisture barrier, the diameter of which, at the welding stage, provides sufficient clearance between the fibers and the heated zone of the welded tube to prevent damage to the fibers due to overheating. The diameter of the formed tube is subsequently reduced by passage through one or more sets of reducing rollers.
Water-blocking means are provided, in the form of a gel, is described, appropriately introduced under pressure through a tube whose outlet is upstream from the welding position.
Cooled gas would also be fed around the fibers and between the fibers and the edges of the folded strip in the welding zone.
U.S. Pat. No. 5,263,239 describes a method for manufacturing an optical cable in which a plurality of optical fibers are embedded in a soft, flexible foam, forming a core, and a metal strip in tubular form shaped around the core, leaving distance between core and tube, welding the tube with a laser device.
The internal diameter of the tube is greater than the cable core so that the core is not damaged by the welding heat.
The welded tube is then drawn down to a diameter close to that of the core by means of a drawing device.
European patent 0023154 describes a cable comprising one or more optical fibers enclosed with a loose-fitting tube which comprises a reinforcing element formed from a metal strip, coated on at least one side with a thermoplastic material, folded into tubular form with overlapping edges, and an extruded plastic sheath.
The heat of extrusion of the plastic sheath causes the fusion of the contacting regions of the thermoplastic coating, sealing the tube. A strip of paper folded in tubular form is arranged around the fibers, with the closed edges of the paper strip diametrically opposite the overlapping edges of the metal tube.
The paper strip folded in tubular form prevents any possible damage to the fibers caused by the inwardly facing edge of the steel strip.
The publication International Wire & Cable Symposium Proceedings 1980, pages 202-210, describes a cable sheath created from a strip of tinned steel, coated with polymer on both faces, 25 mm wide, formed into a tube 7 mm in diameter, with longitudinal overlapping that may be secured during the extrusion of an external polyethylene sheath.
Inside the sheath there is sufficient space to place a longitudinal paper strip and for four cladded fibers 0.5 mm in diameter. The publication reports that a diameter of 7 mm was found to be near the lower limit for forming tubes from strip 150 .mu.m thick by means of a former, since the maximum allowable tensile force would be reduced.
The Applicant notes that reduction of the tube diameter conflicts with the need to utilize support means inside the tube to weld the edges. Without a rigid internal support for the tube, it was not deemed possible to heat-seal the overlapping edges of the tube, due to the need to apply mechanical pressure with sufficient means of thrust.
On the other hand, without effective welding of the tube, it is deemed unlikely that the cable would pass the mechanical tests of repeated torsion-flexure or bending prescribed for the type approval of cables to be installation in pipe. With this in mind, based on the known technique cited above, it is therefore impossible to reduce the diameter value below 7 mm, e.g. diameters of 3-5 mm.
The Applicant has also observed that the need to pass the water infiltration test requires the presence of a predetermined quantity of water-blocking fluid or gel inside the tube enclosing the optical fibers.
In particular, the Applicant has observed that to meet the requirement of ensuring water blockage along the longitudinal axis of the cable, it is desirable to introduce a large quantity of water-blocking substance along with the optical fibers.
For the purposes of this invention, water-blocking material is a material that can be placed within the tube enclosing the optical fibers and prevent the longitudinal passage of water or similar along said tube and, more generally, along the cable.
For purposes of this invention, water-blocking fluid is a material that can be pumped or otherwise fed into the tube enclosing the optical fibers and into other empty spaces inside the cable.
A tube to enclose optical fibers can be made from a metal strip with edges welded, for example, through the action of a laser beam.
During formation of the tube, the optical fibers or ribbons of optical fibers and means for blocking water penetrationin the longitudinal direction, in gel form (water-blocking fluid) can be introduced by means of a small auxiliary pipe. In the case of laser welding, the small pipe serves to protect the optical ribbons from the heat emitted by the laser beam.
If, however, the intention is to produce an optical core of small diameter, significantly smaller than 7 mm, the Applicant has observed that if one attempts to achieve this filling by means of a small-diameter pipe, suitable for feeding the water-blocking material into the tubular core along with the optical fibers, the loss of charge caused by the small diameter of the small tube and the viscosity of the water-blocking fluid makes it necessary to use a high feed pressure for the fluid, necessary to permit feeding of the desired volume of fluid within the pipe.
In fact, the flow velocity of the water-blocking fluid in the small pipe (whose diameter is significantly smaller than that of the tubular core) must be sufficiently high (i.e. rather greater than the speed of advancement of the tubular core being formed) to ensure that a sufficient quantity of water-blocking fluid is fed to fill said tubular core. Said high pressure is such that it causes a high risk of damaging the optical elements and altering their arrangement.
To feed into a pipe 2.75 mm in diameter, for example, a quantity of water-blocking fluid sufficient to fill it by 80%, with a production speed of 15 m/min, using a pipe 1.6 mm in inside diameter and 100 mm long, considering a water-blocking fluid with a viscosity of 70 Pas under application conditions, requires a feed pressure of 270 bars.
Under said conditions, it would be difficult to create a high-pressure fluid feed zone avoiding flow in the opposite direction of the cable advancement and fluid-dynamic interactions with the fibers, a source of significant mechanical stress on the fibers.
In particular, experimental tests conducted with a device for feeding water-blocking fluid comprising a pipe 2 mm in inside diameter and 130 mm long, through which is passed an optical fiber, subjected to a braking of around 100 g and without feed overpressure, have demonstrated that overpressure of around 12 bars result in an increase of around 40 g in the traction applied to the fiber (at a fiber velocity of 1.5 m/min); overpressure of around 28 bars results in an increase of around 550 g in the traction applied to the fiber (at a fiber velocity of 3.5 m/min).
Note also that a total traction of around 650 g applied to a ribbon of four optical fibers corresponds to an elongation of said ribbon greater than 1 per thousand.
As an alternative, to preserve the fibers from the damage resulting from the high pumping pressure of the water-blocking fluid or gel, a lower flow rate may be used, but this results in only partial filling of the unoccupied volume of the tubular core and inadequate resistance of the tube to the passage of water in the longitudinal direction.